The present invention relates to a recording head of an ink jet recording system for providing, on record paper, an output image capable of corresponding to wide needs from the printing industry requesting high-speed output of high quality image, the printer industry based on office or personal request to civil product industry requesting general purpose output apparatus using a variety of kinds and a variety of use of record paper and having low price, and an image recording apparatus utilizing the recording head.
According to a conventional image recording apparatus, particularly, an apparatus using an ink jet system, recording operation is carried out such that droplets of ink are produced and made to fly by an ink jet recording head based on various ink delivering systems (for example, an ink jet recording head by a system of providing mechanical vibration or displacement to ink by using a piezoelectric element or an ink jet recording head by a system of heating to foam ink and utilizing pressure thereof) and portions or a total thereof are adhered onto a recorded medium such as paper. Therefore, in any of the ink jet recording heads, there is formed a nozzle communicating with a delivery port for delivering ink and a single record dot corresponds to a single piece of the nozzle.
Therefore, according to such a conventional image recording apparatus, since the single piece of nozzle of the ink jet recording head for carrying out recording operation by delivering and adhering ink droplets to a recorded medium, corresponds to the single record dot, very small nozzles must be formed over all the width of the ink jet recording head, not only very fine and difficult fabrication technology is needed but also cost of the ink jet recording head is increased and there poses a problem that owing to drying and solidifying ink and a nozzle structure having a small diameter, a jet port is liable to clog and recording operation becomes unstable.
Therefore, there has variously investigated a method of resolving the-problem particular to ink jet recording. Above all, attention is attracted to a slit jet recording system (disclosed in Japanese Patent Publication No.59669/1985) in which an opening portion having a slit-like shape is used and ink is sucked and made to fly from the opening portion by Coulomb""s force since the opening portion is formed in the slit-like shape and therefore, ink is difficult to clog, further, highly integrated formation of the opening portion can be carried out.
Meanwhile, according to the slit jet recording system, since viscous resistance of an ink flow path is low, ink is liable to overflow by collapsing an ink meniscus, further, a shape of a recorded dot is controlled by the ink meniscus.
Therefore, in order to prevent ink hanging at a front end of a head in continuous printing, the front end of the head is subjected to a water repellant processing.
Maximum value p less than max greater than  of back pressure p capable of holding an opening portion in a slit-like shape in the slit jet recording system, is shown as follows by designating a height of a bottom face of a meniscus by notation a(m), surface tension of ink by notation T(N/m), back pressure of ink by notation p(mAq), a density of ink by notation xcfx81 (kg/m3) and the gravitational acceleration by notation g(m/sec2).
p less than max greater than =2T/xcfx81gaxe2x80x83xe2x80x83(1)
Therefore, when the back pressure p exceeds the maximum value, a force exerted to ink functions to balance with the back pressure and therefore, the ink bottom face a spreads by which the meniscus shape is expanded. As a result, an ink dot is enlarged and the resolution is lowered, or when ink overflows from the opening portion and the meniscus collapses, there is produced a nonuniformity of printing caused by occurrence of a failed dot or a dispersion in a dot size, which effects considerable influence on an output of printing.
Further, conversely, when the back pressure p is set to a value sufficiently smaller than the maximum value, an amount of expanding the meniscus is reduced, ink is difficult to fly and blur is liable to cause.
Therefore, in order to provide a stable printing output in the system, it is important to maintain the meniscus uniformly and in a state of being expanded as much as possible and for such purpose, it is necessary to always maintain the back pressure p in a state of being near to the maximum value as much as possible. When a dynamic range in the back pressure adjustment is set to be wide, stability of forming the meniscus is liable to ensure, which amounts to stability of the printing output.
However, in the case of the system of making ink fly by Coulomb""s force as shown in the conventional example, an aqueous species ink having high surface tension cannot be used since voltage drop or electrolysis is caused in the ink jet recording head because the electrical conductivity of the aqueous species ink is high. Therefore, there is used an oil species ink having a low electrical conductivity, in that case, surface tension of the oil species ink is necessarily limited to be low from its physical property and accordingly, the maximum value of the back pressure p is set to be very small from Equation (1). Further, the opening portion of the ink jet recording head is constituted by a slit and therefore, loss by flow path resistance is very small. Therefore, back pressure is directly transmitted as it is and a sensitivity of a change in an amount of the meniscus relative to the back pressure value becomes very high.
When printing is carried out continuously in a state of high back pressure, the meniscus is liable to collapse in a printing area, as a result, overflowed ink may effect adverse influence on image quality.
From the above-described, there pose following problems in the conventional slit jet recording system.
(1) It is difficult to form a proper meniscus shape, as a result of incapable of forming the proper meniscus, the output becomes unstable and image quality is remarkably deteriorated.
(2) Although there is a method of controlling an amount of supplying ink by a small amount by carrying out back pressure control having particularly high accuracy, a control circuit therefor becomes complicated, or a pump having high accuracy is needed and cost of an apparatus therefor is increased.
(3) When the meniscus collapses in a printing area, there poses a problem that overflowed ink causes a failed dot and image quality is deteriorated.
Hence, it is an object of the invention to realize an ink jet recording head for maintaining a meniscus shape of ink always in a constant and proper state without using back pressure controlling means having particularly high accuracy and a recording apparatus capable of outputting an image which is not provided with nonuniformity of printing, is stable and is provided with high image quality thereby.
According to the ink jet recording head and the image recording apparatus using the recording head, front heads of base plates forming an opening portion are formed by a shape of a wedge constituting a shape projected to a recorded medium and critical surface tension at respective. inclined faces of the upper and lower base plates constituting the wedge shape, is made smaller than surface tension of ink. Thereby, a meniscus state of a shape of the meniscus formed to the opening portion is maintained constant. Further, in order to make the critical surface tension of the respective inclined face portions smaller than the surface tension of ink, the respective inclined face portions of the upper and lower base plates constituting the wedge shape, are subjected to an ink repellant processing. Further specifically, the inclined face portions in the wedge shape are subjected to a water repellant or an oil repellant processing in accordance with a material of ink.
Further, portions which are not subjected to the above-described ink repellant processing, for example, outer faces of the base plates communicating with the inclined face portion rearward from the inclined face portions relative to the recorded medium (hereinafter, referred to as head outer faces), are subjected to a hydrophilic processing when the ink is aqueous or a lipophilic processing when the ink is oily.
Or, in order to maintain a state of the meniscus formed at the opening portion constant, there is provided, at least one location, an ink discharging mechanism for discharging extra ink to portions of base plate end face portions constituting the opening portion other than a printing area.
Further, as the ink discharging mechanism, there is provided a notched groove having a size sufficiently larger than an inner width in a short direction of the opening portion at an end face of the base plate at a discharge position. By the constitution, extra ink leaks out to a surface of the base plate always by way of the notched groove portion firstly.
Further, there is constructed a constitution having detecting means for detecting a state of leaking out extra ink at the inclined face portion or the notched groove portion and back pressure controlling means for properly adjusting the back pressure (pressure of supplying ink to the ink jet recording head) in ink supplying means by the detecting means.